1. Field of Invention
This invention relates to optical coherence tomography and more specifically, to an amplified spontaneous emission fiber optic source with high optical power (>20 mW) and a spectral broadband emission (>70 nm) centered near a wavelength of 1060 nm.
2. Description of Related Art
Commercially available retinal ophthalmic optical coherence tomography (OCT) systems operate at a central wavelength of approximately 820 nm due to the relatively low cumulated absorption of the eye tissue at this wavelength. Although the majority of retina imaging reports refer to this band, and ultrahigh resolution has also been demonstrated in this wavelength region for resolving intra-retinal layers, it has limited depth penetration beyond retinal pigment epithelium (RPE). For imaging features beyond the RPE, longer wavelengths are more suitable. See Unterhuber et al., “In vivo retinal optical coherence tomography at 1040 nm-enhanced penetration into the choroids,” Optics Express 13(9), pp. 3252-3258 (2005), the disclosure of which is incorporated by reference herein in its entirety. This relates to the fact that the absorption and scattering properties of melanin (the main chromophore in the RPE) tend to decrease with increasing wavelength.
Water absorption, on the other hand, represents a more critical limitation especially when imaging a biological sample because of its high content (˜90%) of water. There is, however, a spectral window restricted to a wavelength span of 100 nm (a band from 1 μm-1.1 μm) where the water absorption spectrum exhibits a minimum value. Moreover, the optical power loss due to increased water absorption compared to 800 nm band is compensated by the fact that the corneal maximum permissible exposure for longer wavelengths also increases according to American National Standards Institute (ANSI) and International Electrotechnical Commission (IEC) standards. An additional advantage of optical imaging at 1060 nm wavelength band is the zero dispersion point of water, which eliminates the depth dependent broadening of axial resolution over reasonable depth penetration. See Pova{hacek over (z)}ay et al., “Enhanced visualization of choroidal vessels using ultrahigh resolution ophthalmic OCT at 1050 nm,” Optics Express 11, pp. 1980-1986 (2003), the disclosure of which is incorporated by reference herein in its entirety. Recent development of broadband light sources centered at 1060 nm based on amplified spontaneous emission (ASE) from rare-earth doped fibers made possible the development of OCT systems in this band and have proven successful for three dimensional (3D) in vivo retinal imaging with enhanced performance and penetration. See Pova{hacek over (z)}ay et al., “Three-dimensional optical coherence tomography at 1050 nm versus 800 nm in retinal pathologies: enhanced performance and choroidal penetration in cataract patients,” Journal of Biomedical Optics 12(4), 041211-(1-7) (2007); and Yasuno et al., “In vivo high-contrast imaging of deep posterior eye by 1-μm swept source optical coherence tomography and scattering optical coherence angiography”, Optics Express 15(10), pp. 6121-6139 (2007), the disclosures of which are incorporated by reference herein in their entirety. It is predicted that this optical source imaging modality could become a successful alternative to imaging at 820 nm band in early assessments, staging and therapeutic monitoring of retinal diseases.
One prior approach to obtain a broadband spectrum around 1060 nm has been described in U.S. Pat. No. 7,423,803 to Chavez-Pirson et al., the disclosure of which is incorporated by reference herein in its entirety, using a combination of Ytterbium (Yb)-doped fibers only, pumped with a laser diode with the same emission wavelength. This combination relies on the change of the material properties of the co-doping of the fibers. However, their source expands the traditional Yb-doped amplified spontaneous emission (ASE) emission to shorter wavelengths, which will suffer higher optical attenuation due to the 980 nm peak absorption of the water. The valley in the water absorption in centered at ˜1060 nm and measures ˜100 nm at −3 dB point. At an operating central wavelength lower than 1060 nm, the reduction of the signal due to water absorption is more pronounced and leads to a decrease of penetration depth compared to longer wavelengths. See Kou et al., “Refractive indices of water and ice in the 0.65- to 2.5-μm spectral range,” Applied Optics 32(19), pp. 3531-3540 (1993), the disclosure of which is incorporated by reference herein in its entirety.